Systems and methods for digital correction with selective enabling in low intermediate frequency (IF) receivers

ABSTRACT

The embodiments described herein provide systems and methods for digital correction in low intermediate frequency (IF) receivers. Specifically, the embodiments described herein use digital correction techniques that can correct for signal distortions in low IF receivers caused by I-Q imbalance, including both I-Q magnitude imbalance and I-Q phase imbalance. In general, the embodiments described herein are implemented to at least partially cancel an image of a blocking signal in the complex digital signal. Such a cancellation can be implemented to at least partially cancel an image of blocking signal where that image occurs at or near the intermediate frequency. In one embodiment, a corrector is implemented in a low RF receiver and is configured to receive a complex digital signal that includes an image of a blocking signal. Such a low RF receiver can further include a corrector controller to selectively enable the corrector.

TECHNICAL FIELD

The embodiments described herein relate generally to wirelesscommunication, and more particularly, relate to low intermediatefrequency (IF) receivers.

BACKGROUND

Receivers are used in wireless communication to receive wireless signalsand convert those signals into usable information. One type of receiveris generally referred to as a heterodyne receiver. A heterodyne receiveruses frequency mixing to convert the received signals into a newfrequency range for processing. In general, a low intermediate frequency(IF) receiver is a type of heterodyne receiver that mixes the receivedsignals to a non-zero low intermediate frequency (IF). After mixing thereceived signals to the low intermediate frequency the mixed signals canthen be demodulated and the transmitted data extracted. Such low IFreceivers are increasingly used in a wide variety of applications,including short-wavelength digital communication protocols such asBluetooth®.

One continuing issue in some low IF receivers is the presence of signaldistortions due to I-Q imbalance, where this I-Q imbalance can includeboth I-Q magnitude imbalance and I-Q phase imbalance. In general, I-Qimbalance occurs due to mismatches in the receiver chain signal pathsfor the in-phase (I) and quadrature (Q) signals. For example, an I-Qimbalance in a low IF receiver can be created by a mismatch in theanalog gain in the I and Q signal paths. Likewise, an I-Q imbalance canbe created by the use of delay to generate the Q signals that is notexactly 90 degrees. In each case this I-Q imbalance can result inunwanted signal distortions.

Low-IF receivers are especially sensitive to nearby unwantedtransmissions at the image of the IF frequency (i.e., at the −IFlocation). Such signals may be Bluetooth® signals or others frompermitted protocols for the band of operation. Although such signals aretypically unintentional, they may block reception of a wanted signal atthe IF frequency. Such blocking signals created at or near the −IFlocation will be referred to herein as −IF blocking signals.

These −IF blocking signals at the −IF location can be translated throughthe I-Q imbalance and create an unwanted “image” of the blocking signalat the +IF location. Turning now to FIG. 6, a graph 600 illustratesfrequency plots of an exemplary desired signal and a blocking signal.Specifically, the frequency plots can represent an exemplary desiredsignal and blocking signal after mixing in a typical low IF receiver. Aswas described above, a low IF receiver is a type of heterodyne receiverthat mixes received signals to a non-zero low intermediate frequency. Ingraph 600 the desired signal is shown as having a peak at the lowintermediate frequency (i.e., at the +IF location) after mixing.

And as described above, blocking signals may unintentionally interferewith the operation of the receiver. In the example of graph 600 such ablocking signal is shown having a peak near the mirror frequency of theintermediate frequency (i.e., near the −IF location). And as describedabove, this blocking signal at the −IF location can be translatedthrough the I-Q imbalance and create an unwanted “image” of the blockingsignal at the +IF location. In graph 600 an example of such an image ofthe blocking signal is shown at the +IF location.

This image of the −IF blocking signal that is generated at the +IFlocation can be particularity problematic for several reasons.Specifically, because the image of the −IF blocking signal is at the +IFlocation and thus it can be difficult to effectively filter out withoutalso negatively impacting the desired signal. This inability to filterthe out the image of the −IF blocking signal can result in a reductionof the signal-to-noise and distortion ratio (SNDR): in technical termsit will de-sensitize the receiver. Thus, there remains a continuing needfor improved techniques to mitigate the effect of I-Q imbalance.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims when considered inconnection with the Figures, wherein like reference numbers refer tosimilar items throughout the Figures, the Figures are not necessarilydrawn to scale, and:

FIG. 1 shows a schematic diagram of a portion of a low intermediatefrequency (IF) receiver in accordance with an embodiment;

FIG. 2 shows a schematic diagram of a corrector in accordance with anembodiment;

FIG. 3 shows a schematic diagram of a trainer in accordance with anembodiment;

FIG. 4 shows a schematic diagram of a corrector in accordance withanother embodiment;

FIG. 5 shows a graphical representation of a blocking signal and apartially cancelled blocking signal in accordance with an embodiment;

FIG. 6 shows a graphical representation of a desired signal and ablocking signal;

FIG. 7 shows a schematic diagram of a corrector controller in accordancewith another embodiment;

FIG. 8 shows a schematic diagram of an image power calculator inaccordance with another embodiment; and

FIG. 9 shows a schematic diagram of a wideband power calculator inaccordance with another embodiment.

DETAILED DESCRIPTION

The embodiments described herein provide systems and methods for digitalcorrection in low intermediate frequency (IF) receivers that can provideimproved performance. Specifically, the embodiments described herein usedigital correction techniques that can correct for signal distortions inlow IF receivers caused by I-Q imbalance, including both I-Q magnitudeimbalance and I-Q phase imbalance.

In general, the embodiments described herein are implemented to at leastpartially cancel an image of a blocking signal in the complex digitalsignal. Such a cancellation can be implemented to at least partiallycancel an image of blocking signal where that image occurs at or nearthe intermediate frequency (i.e., an image of an −IF blocking signal atoccurs at the IF frequency). As such, the embodiments described hereincan be applied to a wide variety of radio frequency (RF) applications.

In one embodiment, a corrector is implemented in a low RF receiver andis configured to receive a complex digital signal that includes an imageof a blocking signal. Such a corrector can be implemented to at leastpartially cancel the image of the blocking signal using a cancellationsignal.

In some embodiments it can be desirable provide the ability toselectively enable and disable the corrector in response to differentwireless scenarios. For example, in some implementations it can bedesirable to turn on a corrector to produce a cancellation signal onlywhen certain types of unwanted signals are present over the air.Specifically, because a corrector can sometimes introduce artifacts intothe corrected digital signal it can be desirable to only operate thecorrector only when the specific unwanted signals the corrector isimplemented to cancel are present.

For example, in some implementations a corrector that has been trainedto cancel a blocking signal at the image of the IF location (i.e., ablocking signal at the −IF location) can introduce unwanted artifacts inthe corrected complex digital signal if the corrector is active whenthere is no blocking signal at that frequency. This can also be trueeven when there are blocking signals at other nearby frequencies (e.g.,at the −2 IF location, −3 IF location etc.). For all these reasons itcan be desirable in some applications to provide the ability toselectively enable and disable the operation of the corrector dependingon whether a blocking signal is present at the −IF location.

To facilitate this selective enabling of the corrector a correctorcontroller can be provided. In general, the corrector controller can beconfigured to selectively enable the corrector when certain operationalconditions are met. For example, the corrector controller can beconfigured to detect the presence of certain problematic signals andonly enable the corrector when those problematic signals are detected.

In one embodiment, the corrector controller can be configured todetermine measures of power at an image frequency range around the imageof the IF and at a wideband frequency range and to selectively enablethe corrector based at least in part on those determinations.Specifically, the corrector controller can be configured to selectivelyenable the corrector based at least in part on a first measure of powerin the complex digital signal (I+jQ) at an image frequency range and asecond measure of power in the corrected complex digital signal (I+jQ)at a wideband frequency range. In one embodiment the difference betweenthe first measure of power and the second measure of power is determinedand compared to one or more threshold values to determine when thecorrector is enabled and disabled.

As described above, in one embodiment a corrector is implemented in alow RF receiver and is configured to receive a complex digital signalthat includes an image of a blocking signal. Such a corrector can beimplemented to at least partially cancel the image of the blockingsignal using a cancellation signal. Such a low RF receiver can furtherinclude a trainer configured to train the corrector to generate thecancellation signal. In such an embodiment the corrector can use thegenerated cancellation signal to at least partially cancel images ofblocking signals that are at or near the image of the intermediatefrequency.

So implemented, the systems and methods described herein can provideimproved performance in low IF receivers. Specifically, in someembodiments the systems and methods can reduce signal distortions causedby I-Q imbalance and thus increase the signal-to-noise and distortionratio (SNDR) of the low IF receiver.

Turning now to FIG. 1, an exemplary low IF receiver front end 100 inaccordance with the embodiments described herein is illustratedschematically. In general, low IF receivers use a non-zero lowintermediate frequency in the processing of received RF signals.Specifically, low IF receivers use frequency mixing to shift thereceived RF signals to an intermediate frequency as part of a signalprocessing technique that is commonly referred to as heterodyning. Forexample, suitable low IF receivers can shift the received RF signals toan IF frequency of typically a few MHz or lower frequencies. As specificexamples, the low IF frequencies for some applications can be between1-3 MHz, while other applications may use low IF frequencies between100-300 kHz. Those intermediate frequency signals can then be processedand applied to a detector that extracts the transmitted digital data.Such low IF receivers are used in a wide variety of applications,including in digital communication devices that implement a variety ofdigital communication protocols. Finally, it should be noted that FIG. 1is a simplified representation of such a receiver front end, and typicalimplementations of low IF receiver front ends would typically includeadditional elements and features not shown in FIG. 1.

The low IF receiver front end 100 illustrated in FIG. 1 includes an RFinput stage 102, an I-Q mixer 104, an analog-to-digital converter (ADC)106, a corrector 108, a corrector controller 109, a trainer 110, and anoutput stage 112. In general, the low IF receiver front end 100 receivesRF signal energy from an antenna, processes the RF signal energy toextract digital data from the RF signal energy, and outputs the digitaldata to the remaining elements of the physical layer (e.g., acquisitionelements and demodulation elements) in whatever system or device isimplemented to receive the digital data. The low IF receiver front end100 is an example of a receiver front end that uses low IF signalprocessing techniques, and thus can be implemented in a variety ofdevices and used in a variety of applications for which low IFarchitectures are appropriate. For example, the low IF receiver frontend 100 can be implemented for use in digital communication devices inBluetooth® wM-Bus®, SigFox®. Z-Wave®, 802.15.4 and other communicationstandards.

In this illustrated embodiment the RF input stage 102 receives signalenergy from the antenna, amplifies the received signal energy, andpasses the amplified signals to the I-Q mixer 104. The signal energyreceived by the RF input stage 102 includes quadrature signals. Thus,the signal energy includes signals in both I and Q channels where the Iand Q channels have a quadrature (i.e., 90 degree) relative phasedifference.

In a typical implementation the RF input stage 102 could include asuitable impedance matching network coupled to the antenna and asuitable low noise amplifier (LNA). Other elements in the RF input stage102 could include power detectors, filters and other gain controlstages. As will be described in greater detail below, in the embodimentsdescribed herein the RF input stage 102 is also configured to receive atraining signal from the trainer 110, where the training signal will beused to train the corrector 108.

The I-Q mixer 104 receives the amplified signals from the RF input stage102 and down mixes both the I and Q channels to a suitable lowintermediate frequency. To facilitate this the I-Q mixer 104 usesquadrature mixing signals (i.e., separate I and Q mixing signals)implemented to ideally have an identical frequency and a precise90-degree relative phase shift. In one embodiment, these separate I andQ mixing signals can be generated through a voltage-controlledoscillator (VCO) controlled by a phase-locked loop to generatequadrature (90-degree) components.

The I-Q mixer 104 thus will generate I and Q output signals, with eachof the I and Q output signals having a frequency centered at the lowintermediate frequency and having a phase difference of approximately 90degrees. In some embodiments these I and Q output signals can befiltered or otherwise processed. For example, these I and Q outputsignals can be applied to a suitable complex band pass filter configuredto filter around the intermediate frequency. These I and Q outputsignals can be then be passed to the ADC 106.

It should be noted in an ideal implementation the I and Q mixing signalswould be separated by precisely 90 degrees. Furthermore, the resulting Iand Q output signals of the I-Q mixer would also ideally have equalamplitudes. Furthermore, any filters or other elements would also beideally be precisely matched. However, in real world implementationsthere will always be some phase error and amplitude mismatch, with theresult of this phase error and amplitude mismatch being an I-Q imbalancein the I and Q output signals that are applied to the ADC 106.

The ADC 106 is configured to receive the I and Q output signals andconvert the I and Q output signals to a complex digital signal, wherethe complex digital signal is in the form of (I+jQ). As such, the ADC106 can be implemented with any suitable analog-to-digital conversiondevice or technique. For example, the ADC 106 can be implemented with aNyquist 10-bit (for example) converter based on a SAR architecture.After some processing (such as decimation and DC offset cancellation,the complex digital signal (I+jQ) is then outputted to the corrector108.

In general, the corrector 108 is implemented to receive the complexdigital signal (I+jQ) and at least partially cancel an image of ablocking signal in the complex digital signal (I+jQ). Specifically, thecorrector 108 is configured to at least partially cancel an image of ablocking signal in the complex digital signal (I+jQ) by generating anappropriate cancellation signal. This cancellation signal, when appliedto the complex digital signal (I+jQ), will at least partially cancel theimage of the blocking signal. The corrector 108 thus generates acorrected complex digital signal which is outputted to the output stage112. Several detailed examples of how the corrector 108 can beimplemented will be discussed below with reference to FIGS. 2-5.

In general, the corrector controller 109 is configured to selectivelyenable the corrector 108 when certain operational conditions are met.For example, the corrector controller 109 can be configured to detectthe presence of certain problematic signals and only enable thecorrector when those problematic signals are detected. In oneembodiment, the corrector controller 109 can be configured to determinemeasures of power at an image frequency range around the image of the IFand at a wideband frequency range and to selectively enable thecorrector based at least in part on those determinations. Specifically,the corrector controller 109 can be configured to selectively enable thecorrector based at least in part on a first measure of power in thecomplex digital signal (I+jQ) at an image frequency range and a secondmeasure of power in the corrected complex digital signal (I+jQ) at awideband frequency range. In one embodiment the difference between thefirst measure of power and the second measure of power is determined andcompared to one or more threshold values to determine when the correctoris enabled and disabled. Examples of such a corrector controller 109will be discussed in greater detail with reference to FIGS. 7-9.

In general, the trainer 110 is configured to train the corrector 108 toproduce the cancellation signal needed to cancel an image of a blockingsignal and generate a corrected complex digital signal. To facilitatethis, the trainer 110 is coupled to both the input stage 102 and thecorrector 108. During training the trainer 110 generates a trainersignal that is used to train the corrector 108. For example, the trainer110 can be configured to generate and apply the training signal to theRF input stage 102. From there, the training signal propagates throughthe I-Q mixer 104 and the ADC 106 and is applied to the corrector 108 asa complex digital training signal. There, the complex digital trainingsignal is used to train the corrector 108. Several detailed examples ofsuch a training procedure will be described in greater detail below.

As described above, the corrector 108 generates a corrected complexdigital signal which is outputted to the output stage 112. In general,the output stage 112 can be implemented to perform any additional outputprocessing of the corrected complex digital signal needed by the low IFreceiver front end 100. Thus, the particular processing operationsperformed by the output stage 112 would depend on the application forthe low IF receiver front end 100 is implemented. As some examples, theoutput stage 112 can typically include a digital intermediate frequencymixer that is used to downmix the corrected complex digital signal tothe baseband frequency. When downmixed to baseband frequency the complexdigital signal can then be demodulated and processed to extract thedigital data. To facilitate this extraction of the digital data theoutput stage 112 can include additional processing elements such as anautomatic digital gain control decimation, down mixing, and baseband andchannel filtering. The extracted digital data can then be outputted towhatever system or device is implemented to receive the digital data.

In some embodiments, the corrector 108 includes a complex gain adjuster,and the trainer 110 includes a complex gain trainer configured to trainthe complex gain adjuster. As will be described in greater detail below,this complex gain adjuster can be implemented with a multiplier used toimplement the selected gain. Examples of such embodiment will bediscussed below with reference to FIGS. 2 and 5.

Also, in such an embodiment the complex gain trainer can comprise asquaring function, a gain function and an accumulator configured toreceive an output of the corrector 108, and where an output of theaccumulator is provided to configure an amount of complex gain providedby the complex gain adjuster based at least in part on the output of thecorrector 108. A detailed example of such an embodiment will bediscussed with reference to FIG. 3 below.

In one embodiment, the corrector 108 includes a complex conjugateoperator and a predictor following the complex conjugate operator in thecorrector 108. In such an embodiment, the trainer 110 can include atraining signal generator and a predictor trainer. In such an embodimentthe training signal generator is configured to apply a training signalto the RF input stage 102 and the predictor trainer is configured totrain the predictor with the training signal applied to the RF inputstage 102. An example of such an embodiment will be described in greaterdetail below with reference to FIG. 2. In one such embodiment thepredictor can be implemented with a complex filter and an adjustabledelay.

In one embodiment, the predictor includes a complex filter and anadjustable delay. Specifically, the complex filter can be implementedwith a complex finite impulse response (FIR) filter having two taps orthree taps.

In another embodiment, the corrector 108 includes a complex conjugateoperator and an equalizer preceding the complex conjugate operator inthe corrector 108. In such an embodiment, the trainer 110 can include atraining signal generator and an equalizer trainer. In such anembodiment the training signal generator is configured to apply atraining signal to the RF input stage 102 and the equalizer trainer isconfigured to train the equalizer with the training signal applied tothe RF input stage 102. An example of such an embodiment will bedescribed in greater detail below with reference to FIG. 4.

In one embodiment, the equalizer includes a complex filter and anadjustable delay. Specifically, the complex filter can be implementedwith a complex finite impulse response (FIR) filter having two taps orthree taps.

Turning now to FIG. 2, an exemplary corrector 200 and trainer 210 inaccordance with the embodiments described herein is illustratedschematically. The corrector 200 is an example of the type of correctorthat can be used in low IF receiver front end 100 illustrated in FIG. 1.As was described above, the corrector 200 is implemented receive acomplex digital signal (I+jQ) and at least partially cancel an image ofa blocking signal in the complex digital signal by generating anappropriate cancellation signal. This cancellation signal, when appliedto the complex digital signal, will at least partially cancel the imageof the blocking signal. The corrector 200 thus through imagecancellation generates a corrected complex digital signal.

Likewise, the trainer 210 is an example of the type of trainer that canbe used in low IF receiver front end 100 illustrated in FIG. 1. Again,the trainer 210 is configured to train the corrector 200 to produce thecancellation signal needed to cancel an image of a blocking signal andgenerate a corrected complex digital signal.

In the example of FIG. 2 the corrector 200 includes a complex conjugateoperator 202, a predictor 204, a complex gain adjuster 206 and an imagecanceller 208. Notably, in this embodiment the complex conjugateoperator 202 comes before the predictor 204. The trainer 210 in thisexample includes a predictor trainer 212 and a complex gain trainer 214.

In general, the predictor 204 and the complex gain adjuster 206 are bothused together to generate a cancellation signal that will reduce theeffects of distortion in the complex digital signal by cancelling theimage of a blocking signal in a complex digital signal. However, thepredictor 204 and the complex gain adjuster 206 are implemented to usedifferent techniques and to be trained at different intervals.Specifically, the predictor 204 is implemented with computationallyintensive complex filtering techniques that can require significant timeto train. Thus, the predictor 204 is implemented to be trainedperiodically when the device is offline or otherwise not being used forreceiving RF signals. For example, the predictor 204 can be implementedto be trained only once (e.g., at initial device assembly or first startup) or at predetermined intervals (e.g., during selected devicerestarts).

In contrast, the complex gain adjuster 206 is implemented to use lesscomputationally intensive gain adjustment techniques and can be trainedmuch faster than the predictor. This allows the complex gain adjuster206 to adjust and optimize the cancellation signal in response tofrequent operational changes. For example, the complex gain adjuster 206can be configured to train before the arrival of every packet tooptimize the cancellation signal. Thus, taken together the predictor 204and the complex gain adjuster 206 can provide an effective cancellationsignal for cancelling the image of a blocking signal in a complexdigital signal.

During operation, the complex conjugate operator 202 receives thecomplex digital signal and generates the complex conjugate of thesignal. Described mathematically, this operation changes the sign of theQ component of the complex digital signal. This operation generates animage frequency much alike the undesired distortion. The complexconjugate of the complex digital signal is then passed to the predictor204.

In general, the predictor 204 receives the complex conjugate and filtersfrom the complex conjugate the effects of distortion resulting from theimage of the −IF blocking signal. To perform this filtering thepredictor 204 can be implemented with a complex filter and an adjustabledelay. In general, the complex filter and adjustable delay areimplemented to shape and delay-match the conjugated signal in order togenerate a cancellation signal of opposite polarity to the image of the−IF blocking signal. In such an embodiment the parameters of the complexfilter and the adjustable delay are determined during off-line trainingto generate a cancellation signal from the complex conjugate that can beused to cancel the distortions caused by the image of the −IF blockingsignal. It should be noted that this training is done solely with theuse of a training signal provided by the trainer 210, as will bedescribed in greater detail below.

In one embodiment, the complex filter in the predictor 204 isimplemented with a complex finite impulse response (FIR) filter, wherethe coefficients of the FIR filter are complex numbers. For example, thecomplex filter can comprise a two-tap complex FIR filter that includestwo complex coefficients. Such a two-tap complex FIR filter can beimplemented to effectively generate the cancellation signal whencombined with the adjustable delay. In other embodiments, a three-tapcomplex FIR filter can instead be used, although such an embodiment willrequire significantly more time to train the filter and will increasepower consumption. Of course, these are just non-limiting examples, andother types of complex filters can be used to implement the predictor204.

In one embodiment, the complex filter is implemented to have a near zerobetween the IF frequency and zero frequency and a characteristic V-shapein the region of interest. Note that such a complex filter will alsocreate artifacts for blockers away from the −IF region, such artifactscan be minimized through further processing of the cancellation signalthrough a filter centered at the IF frequency.

In general, the adjustable delay in the predictor 204 is provided tomake the cancellation signal time-matched with the complex digitalsignal at the image canceller 208. The adjustable delay when chosencarefully can simplify the implementation of the predictor 204.Specifically, the use of such an adjustable delay can greatly reduce thenumber of coefficients required in the complex filter, and thus cangreatly simplify the training of the complex filter. In principle, theamount of delay is configured as to compensate for the group delay ofthe front-end analog filters. Specifically, the amount of delay providedby the adjustable delay would be determined during training of thepredictor 204.

The output of the predictor 204 is passed to the complex gain adjuster206. In general, the complex gain adjuster 206 is implemented to providea relatively fine gain and phase adjustment to the cancellation signalduring operation of the low IF receiver. Thus, the complex gain adjuster206 can provide a continuous, real time adjustment to the cancellationsignal in a way that can compensate for environmental and other changesto the system. In one embodiment, the complex gain adjuster 206 can beimplemented with a digital complex multiplier that multiplies theincoming cancellation signal by a complex value. This digital complexmultiplier can thus provide a selected amount of magnitude and gaincontrol to the incoming cancellation signal, and that amount can beretrained relatively often. For example, the amount of gain can beretrained before turning on the reception of an expected incoming packetif environmental conditions have changed or enough time has passed.

The output of the complex gain adjuster 206 is provided to the imagecanceller 208. There, the cancellation signal generated by the complexconjugate operator 202, the predictor 204 and the complex gain adjuster206 is combined with the original complex digital signal to generate acorrected complex digital signal. For example, the cancellation signalcan be summed to the complex digital signal using a complex summer togenerate the corrected complex digital signal. This corrected complexdigital signal can then be provided to the output stage (e.g., outputstage 112, FIG. 1).

As noted above, in this embodiment the complex conjugate operator 202comes before the predictor 204. This configuration facilities relativelyeasy training of the predictor 204 to provide the desired correction. Itshould be noted that the conjugate function provided by the complexconjugate operator 202 is a very nonlinear function which “flips” thespectrum in order to prepare the signal for the cancellation processprovided by the predictor 204. This nonlinearity prevents the complexconjugate operator 202 from being commuted with the predictor 204 bychanging their relative position in the corrector 200 without asignificant change to the operation of the corrector 200. Instead, whenthe position of the complex conjugate operator 202 is commuted the“predictor” will not function as a predictor can instead be viewed tofunction as a front-end filter equalizer. Such an embodiment will bedescribed in greater detail below with reference to FIG. 4.

As was described above, the predictor 204 and the complex gain adjuster206 are used together to generate a cancellation signal that can be usedto reduce the effects of distortion in the complex digital signalcancelling the image of a blocking signal in a complex digital signal.And again, the predictor 204 and the complex gain adjuster 206 areimplemented to use different techniques and to be trained at differentintervals. To facilitate these different training techniques the trainer210 includes a predictor trainer 212 and a complex gain trainer 214.

The predictor trainer 212 is coupled to the predictor 204 and the RFinput stage (e.g., RF input stage 102, FIG. 1). As described above, thepredictor 204 is implemented with computationally intensive complexfiltering techniques that can require significant time to train, andthus the predictor trainer 212 is applied relatively rarely (e.g.,during an initialization procedure) or at predetermined intervals (e.g.,during selected device restarts) to train the predictor 204 when the lowIF receiver is offline or otherwise not being used for receiving RFsignals.

Thus, when the low IF receiver is not being used for receiving RFsignals the predictor trainer 212 can generate a training signal andapply that training signal to the input stage of the low IF receiverfront end. As one example, the training signal can be generated by anauxiliary phase locked loop in the predictor trainer 212 and thenprovided to a low noise amplifier (LNA) in the RF input stage. At thesame time the main input to the LNA can be set to a high impedance suchthat only the training signal is amplified by the LNA. The effects ofthis amplified training signal will then propagate through the low IFreceiver front end (e.g., through the I-Q mixer 104, ADC 106) until theyreach the corrector (e.g., corrector 108, 200). There, the trainingsignal can be used to train the predictor 204.

An example training procedure performed by the predictor trainer 212 isas follows. First, the training signal received at the input of thepredictor 204 is saved off-line. Second, the saved training signal andits conjugate are then filtered with a high order filter to de-emphasizethe components at −IF. Third, the filtered training signal and thefiltered conjugate of the training signal can then be used todetermining the coefficients of the complex filter response equation.For example, in the cased of a FIR complex filter the filter responsex(n) can be defined as:x(n)=a ₀ xc(n−k)+a ₁ xc(n−1−k)+a ₂ xc(n−2−k)+ε_(n)where a₀, a₁, and a₂ are complex coefficients and the k is an offsetvalue that represents the group delay. According to this method, thecomplex coefficients a₀, a₁, and a₂ can be determined with aleast-squares fit using the filtered training signal as output and thefiltered conjugate as input. Likewise, the time offset value k can beoptimized with trial and error by doing the least-squares fit forseveral values of k and choosing the k that gives the best performanceoverall. Once so determined the complex coefficients and the k value canbe saved and used by the predictor 204. Note that while the filterresponse equation above shows three coefficients (a₀, a₁, and a₂), otherembodiments may use a different number of coefficients to provide a goodfit.

The complex gain trainer 214 is likewise coupled to the complex gainadjuster 206 and the output of the image canceller 208. As describedabove, the complex gain adjuster 206 is implemented to use lesscomputationally intensive gain adjustment techniques and can train muchfaster and more often. This allows the complex gain adjuster 206 toadjust and optimize the cancellation signal in response to changes dueto temperature and aging. It should be noted that the complex gainadjuster 206 is thus trained only after the predictor 204 has beentrained. Thus, the complex gain adjuster 206 can be used to maintain thecancellation signal near optimal with environmental changes while thepredictor 204 values predict intrinsic behavior that does not need asmuch adaptation.

The complex gain trainer 214 is coupled to the complex gain adjuster 206and the output of the image canceller 208. In general, the complex gaintrainer 214 is based on the DC component cancellation of the square ofthe output of the image canceller. The simplicity and stability of thisalgorithm facilitates relatively quick training that can be repeatedlyperformed as environmental conditions change. This quick adaptation maybe done by waking up the receiver slightly earlier then required andperforming a quick adaptation before allowing the reception of externalsignals.

Turning now to FIG. 3, one embodiment of complex gain trainer 300 isillustrated in greater detail. The complex gain trainer 300 is anexample of the type of complex gain trainer that can be implemented inthe trainer (e.g., trainer 110, 210, 410). In this embodiment thecomplex gain trainer 300 includes a squaring function 302, a gainfunction 304, and an accumulator 306. The complex gain trainer 300receives the corrected complex digital signal from the output of theimage canceller (e.g., image canceller 208) and outputs to the complexgain adjuster (e.g., complex gain adjuster 206). It should be noted thatthe complex gain trainer 300 is architected as part of a feedback loopthat is designed to minimize DC components in the feedback loop andtrains the complex gain adjuster to cancel the image of the blocker andmitigate the effects of IQ imbalance.

Specifically, the complex gain trainer 300 receives a feedback of thecorrected complex digital signal from the image canceller at thesquaring function 302. The squaring function 302 squares this correctedcomplex digital signal in the complex domain. Because the squaringfunction 302 performs the squaring in the complex domain it generates a2*IF component and a DC component. The squared corrected complex digitalsignal is then passed to the gain function 304 and then to theaccumulator 306.

In general, the squared corrected complex digital signal is accumulatedby the accumulator 306 while the gain function 304 controls theadaptation rate of the feedback loop. The cancellation loop uses thiscomplex value accumulator 306 as an integrative control to accuratelygenerate a cancellation signal for image of the −IF blocking signal. Inthis operation the accumulator 306 effectively filters out the 2*IFcomponent and integrates the DC component.

Note that the cancellation loop can be made stable by keeping the loopgain low enough. Also, as the cancellation loop is a relatively simplefirst order loop it can train the complex gain adjuster relatively fast.

Returning to FIG. 2, the corrector 200 so implemented and trained withthe trainer 210 can thus receive a complex digital signal and at leastpartially cancel an image of a blocking signal in the complex digitalsignal by generating an appropriate cancellation signal. And thus thecorrector 200 can provide improved performance in low IF receivers byreducing signal distortions caused by I-Q imbalance and thus increasethe signal-to-noise and distortion ratio (SNDR) during normal packetreception.

Turning now to FIG. 4, another exemplary corrector 400 and trainer 410in accordance with the embodiments described herein is illustratedschematically. The corrector 400 is another example of the type ofcorrector that can be used in low IF receiver front end 100 illustratedin FIG. 1. In general, the corrector 400 differs from the corrector 200in FIG. 2 in that it utilizes an equalizer before the complex conjugateoperator rather than a predictor after the complex conjugate operator.

Like the corrector 200, the corrector 400 is implemented receive acomplex digital signal (I+jQ) and at least partially cancel an image ofa blocking signal in the complex digital signal by generating anappropriate cancellation signal. This cancellation signal, when appliedto the complex digital signal, will at least partially cancel the imageof the blocking signal.

Likewise, the trainer 410 is an example of the type of trainer that canbe used in low IF receiver front end 100 illustrated in FIG. 1. Again,the trainer 410 is configured to train the corrector 400 to produce thecancellation signal needed to cancel an image of a blocking signal andgenerate a corrected complex digital signal.

In the example of FIG. 4 the corrector 400 includes an equalizer 402, acomplex conjugate operator 404, a complex gain adjuster 406 and an imagecanceller 408. Notably, in this embodiment the equalizer 402 comesbefore the complex conjugate operator 404. The trainer 410 in thisexample includes an equalizer trainer 412 and a complex gain trainer414.

In general, the equalizer 402 and the complex gain adjuster 406 are bothused together to generate a cancellation signal that will reduce theeffects of distortion in the complex digital signal by cancelling theimage of a blocking signal in a complex digital signal. However, likethe predictor 204 described above, the equalizer 402 is implemented touse different techniques and to be trained at different intervalscompared to the complex gain adjuster 406. Specifically, the equalizer402 is implemented with computationally intensive complex filteringtechniques that can require significant time to train. Thus, theequalizer 402 is implemented to be trained periodically when the deviceis offline or otherwise not being used for receiving RF signals. Forexample, the equalizer 402 can be implemented to be trained only once(e.g., during an initialization procedure) or at predetermined intervals(e.g., during selected device restarts).

In contrast, the complex gain adjuster 406 is again implemented to useless computationally intensive gain adjustment techniques and can betrained much faster than the predictor. This allows the complex gainadjuster 406 to adjust and optimize the cancellation signal in responseto frequent operational changes. For example, the complex gain adjuster406 can be configured to train before the arrival of every packet tooptimize the cancellation signal. Thus, taken together the equalizer 402and the complex gain adjuster 406 can provide an effective cancellationsignal for cancelling the image of a blocking signal in a complexdigital signal.

In general, the equalizer 402 receives the complex digital signalfilters from the complex digital signal the effects of distortionresulting from the image of the −IF blocking signal. To perform thisfiltering the equalizer 402 can be implemented with a complex filter andan adjustable delay. In general, the complex filter and adjustable delayare again implemented to shape and delay-match the conjugated signal inorder to generate a cancellation signal of opposite polarity to theimage of the −IF blocking signal. In such an embodiment the parametersof the complex filter and the adjustable delay are determined duringtraining to generate a cancellation signal from the complex digitalsignal that can be used to cancel the distortions caused by the image ofthe −IF blocking signal. It should be noted that this training is donesolely with the use of a training signal provided by the trainer 410, aswill be described in greater detail below.

In one embodiment, the complex filter in the equalizer 402 isimplemented with a complex finite impulse response (FIR) filter, wherethe coefficients of the FIR filter are complex numbers. For example, thecomplex filter can comprise a two-tap complex FIR filter that includestwo complex coefficients. Such a two-tap complex FIR filter can beimplemented to effectively generate the cancellation signal while notrequiring an excessive amount of training time. In other embodiments, athree-tap complex FIR filter can instead be used, although such anembodiment will require significantly more time to train the filter andwill increase power consumption. Of course, these are just non-limitingexamples, and other types of complex filters can be used to implementthe equalizer 402.

In one embodiment, the complex filter is implemented to have a near zerobetween the IF frequency and zero frequency and a characteristic V-shapein the region of interest. Note that such a complex filter will alsocreate artifacts for blockers away from the −IF region, such artifactscan be minimized through further processing of the cancellation signalthrough a filter centered at the IF frequency.

In general, the adjustable delay in the equalizer 402 is provided tomake the cancellation signal time matched with the complex digitalsignal at the image canceller 408. The use of such an adjustable delaycan simplify the implementation of the equalizer 402. Specifically, theuse of such an adjustable delay can greatly reduce the number ofcoefficients required in the complex filter, and thus can greatlysimplify the training of the complex filter. In principle, the amount ofdelay is configured is to compensate for the group delay of the frontend analog filters. Specifically, the amount of delay provided by theadjustable delay would be determined during training of the equalizer402.

The output of the equalizer 402 is passed to the complex conjugateoperator 404, which generates the complex conjugate of the signal.Described mathematically, this operation changes the sign of the Qcomponent of the complex digital signal. This operation generates animage frequency much alike the undesired distortion. The complexconjugate of the complex digital signal is then passed to the complexgain adjuster 406.

In general, the complex gain adjuster 406 is again implemented toprovide a relatively fine gain and phase adjustment to the cancellationsignal during operation of the low IF receiver. Thus, the complex gainadjuster 406 can provide a continuous, real time adjustment to thecancellation signal in a way that can compensate for environmental andother changes to the system. In one embodiment, the complex gainadjuster 406 can be implemented with a digital complex multiplier thatmultiplies the incoming cancellation signal by a complex value. Thisdigital complex multiplier can thus provide a selected amount ofmagnitude and gain control to the incoming cancellation signal, and thatamount can be retrained relatively often. For example, the amount ofgain can be retrained before turning on the reception of an expectedincoming packet if environmental conditions have changed or enough timehas passed.

The output of the complex gain adjuster 406 is provided to the imagecanceller 408. There, the cancellation signal generated by the equalizer402, the complex conjugate operator 404, and the complex gain adjuster406 is combined with the original complex digital signal to generate acorrected complex digital signal. For example, the cancellation signalcan be summed to the complex digital signal using a complex summer togenerate the corrected complex digital signal. This corrected complexdigital signal can then be provided to the output stage (e.g., outputstage 112, FIG. 1).

As noted above, in this embodiment the equalizer 402 comes before thecomplex conjugate operator 404. It should again be noted that theconjugate function provided by the complex conjugate operator 404 is avery nonlinear function which “flips” the spectrum after thecancellation process provided by the equalizer 402. This nonlinearity,again, prevents the complex conjugate operator 404 from being commutedwith the equalizer 402 by changing their relative position in thecorrector 400 without a significant change to the operation of thecorrector 400. While this embodiment can provide effective cancellationwhen properly implemented, it may also be significantly more difficultto train the equalizer 402 compared to the training of the predictor 204described above.

As was described above, the equalizer 402 and the complex gain adjuster206 are used together to generate a cancellation signal that can be usedto reduce the effects of distortion in the complex digital signalcancelling the image of a blocking signal in a complex digital signal.And again, the equalizer 402 and the complex gain adjuster 206 areimplemented to use different techniques and to be trained at differentintervals. To facilitate these different training techniques the trainer410 includes an equalizer trainer 412 and a complex gain trainer 414.

The equalizer trainer 412 is coupled to the equalizer 402 and the RFinput stage (e.g., RF input stage 102, FIG. 1). As described above, theequalizer 402 is implemented with computationally intensive complexfiltering techniques that can require significant time to train, andthus the equalizer trainer 412 is applied relatively rarely (e.g., onlyon initial setup) to train the equalizer 402 when the low IF receiver isoffline or otherwise not being used for receiving RF signals.

Thus, when the low IF receiver is not being used for receiving RFsignals the equalizer trainer 412 can generate a training signal andapply that training signal to the input stage of the low IF receiverfront end. As one example, the training signal can be generated by anauxiliary phase locked loop in the equalizer trainer 412 and thenprovided to a low noise amplifier (LNA) in the RF input stage. At thesame time the main input to the LNA can be set to a high impedance suchthat only the training signal is amplified by the LNA. The effects ofthis amplified training signal will then propagate through the low IFreceiver front end (e.g., through the I-Q mixer 104, ADC 106) until theyreach the corrector (e.g., corrector 108, 400). There, the trainingsignal can be used to train the equalizer 402.

The training procedure performed by the equalizer trainer 412 canclosely track the procedure performed by the predictor trainer 212 thatwas described above. However, in this embodiment a result of thetraining signal is extracted from an internal input of the equalizer 402and both the training signal and the extracted result of the trainingsignal are both saved to use in training.

Specifically, the training signal is again applied to the RF inputstage. The result of the training signal propagating through the RFinput stage is then extracted at an internal input to the equalizer 402.For example, the result of training signal at the internal input to theequalizer 402 can be sampled through lab instrumentation or reproducedsynthetically. The training signal and the result of the training signalat the internal input to the equalizer are both saved off-line. Second,the saved training signal and the saved result of the training signalare then filtered with a high order filter to de-emphasize thecomponents at the +IF location. Third, the saved training signal andsaved result of the training signal can then be used to determining thecoefficients of the complex filter response equation, as was describedabove. For example, the complex coefficients can be determined with aleast-squares fit using the saved results of the training signal at theequalizer input as an input to the least-squares fit, and using thesaved training signal as an output to the least squares fit. Likewise,the time offset value can be optimized with trial and error by doing theleast-squares fit for several values choosing the offset value gives thebest performance overall.

In this embodiment the complex gain trainer 414 can function in the samemanner as the complex gain trainers 214 and 300 described above.Specifically, the complex gain trainer 414 is similarly coupled to thecomplex gain adjuster 406 and the output of the image canceller 408. Asdescribed above, the complex gain adjuster 406 is implemented to useless computationally intensive gain adjustment techniques and can trainmuch faster and more often. This allows the complex gain adjuster 406 toadjust and optimize the cancellation signal in response to changes dueto temperature and aging. It should be noted that the complex gainadjuster 406 is thus trained only after the equalizer 402 has beentrained. Thus, the complex gain adjuster 406 can be used to maintain thecancellation signal near optimal with environmental changes while theequalizer 402 values predict intrinsic behavior that does not need asmuch adaptation.

As one specific example, the complex gain trainer 414 can be implementedwith a squaring function 302, gain function 304, and accumulator 306, aswas described above with reference to FIG. 3. In this embodiment thecomplex gain trainer 414 can operate by receiving a feedback of thecorrected complex digital signal from the image canceller 408 andsquaring this corrected complex digital signal in the complex domain.The squared corrected complex digital signal is then accumulated by theaccumulator while the gain function controls the adaptation rate of thisaccumulation. The result of this accumulation thus can be used to setthe gain of the complex gain adjuster 406 to more accurately generate acancellation signal for image of the −IF blocking signal.

Thus, the corrector 400 can be implemented and trained to at leastpartially cancel an image of a blocking signal in the complex digitalsignal by generating an appropriate cancellation signal. And thus thecorrector 400 can provide improved performance in low IF receivers byreducing signal distortions caused by I-Q imbalance and thus increasethe signal-to-noise and distortion ratio (SNDR).

Turning now to FIG. 5, a graph 500 illustrates an exemplary frequencyresponse graph of an exemplary blocking signal before and after blockingsignal cancellation using a corrector as described herein (e.g.,corrector 108, 200, 400). As can be seen in graph 500, the corrector iseffectively cancelling at least part of an image of a blocking signal atthe IF location. Specifically, in this example the corrector is able toachieve approximately 16 dB of reduction in the image of the blockingsignal at the IF location. Again, such a reduction can significantlyincrease the signal-to-noise and distortion ratio (SNDR) of the signalsreceived by the low IF receiver.

In some embodiments it can be desirable provide the ability toselectively enable and disable the corrector (e.g., corrector 108, 200,400) in response to different wireless scenarios. For example, in someimplementations it can be desirable to turn on a corrector to produce acancellation signal only when certain types of unwanted signals arepresent over the air. Specifically, because a corrector can sometimesintroduce artifacts into the corrected digital signal it can bedesirable to only operate the corrector only when the specific unwantedsignals the corrector is implemented to cancel are present.

For example, in some implementations a corrector (e.g., corrector 108,200, 400) that has been trained to cancel a blocking signal at the imageof the IF location (i.e., a blocking signal at the −IF location) canintroduce unwanted artifacts in the corrected complex digital signal ifthe corrector is active when there is no blocking signal at thatfrequency. This can also be true even when there are blocking signals atother nearby frequencies (e.g., at the −2 IF location, −3 IF locationetc.). For all these reasons it can be desirable in some applications toprovide the ability to selectively enable and disable the operation ofthe corrector depending on whether a blocking signal is present at the−IF location.

To facilitate this selective enabling of the corrector a correctorcontroller can be provided. In general, the corrector controller can beconfigured to selectively enable the corrector when certain operationalconditions are met. For example, the corrector controller can beconfigured to detect the presence of certain problematic signals andonly enable the corrector when those problematic signals are detected.

In one embodiment, the corrector controller can be configured todetermine measures of power at an image frequency range around the imageof the IF and at a wideband frequency range and to selectively enablethe corrector based at least in part on those determinations.Specifically, the corrector controller can be configured to selectivelyenable the corrector based at least in part on a first measure of powerin the complex digital signal (I+jQ) at an image frequency range and asecond measure of power in the corrected complex digital signal (I+jQ)at a wideband frequency range. In one embodiment the difference betweenthe first measure of power and the second measure of power is determinedand compared to one or more threshold values to determine when thecorrector is enabled and disabled.

Turning now to FIG. 7, a schematic diagram of an exemplary correctorcontroller 700 is illustrated. The corrector controller 700 includes animage power calculator 702, a wideband power calculator 704, acomparator 706, and a control signal generator 708. In general, thecorrector controller 700 is configured to selectively enable thecorrector based at least in part on a first measure of power determinedby the image power calculator 702 and a second measure of the powerdetermined by the wideband power calculator 704. Specifically, thedifference between the first measure of power and the second measure ofpower is determined by the comparator 706, and this difference is usedby the control signal generator 708 to determine when to enable anddisable the corrector.

The image power calculator 702 is configured to receive an uncorrecteddigital signal generate a first measure of power in the complex digitalsignal at an image frequency range around an image frequency of the lowintermediate frequency. As one example, the image power calculator 702can be configured to determine a measure of power at a frequency rangeapproximately equal to the signal bandwidth and centered at the image ofthe intermediate frequency (i.e., at the −IF location).

As will be described in greater detail below, the power at such an imagefrequency of the IF is indicative of the presence or absence of ablocking signal at the −IF location. A variety of different types ofdevices can be used to determine such a first measure of power. In oneexample, magnitude detector and an averaging filter are used todetermine the first measure of power. In this example the magnitudedetector serves to determine a magnitude of the mixed complex digitalsignal and the averaging filter serves to average the determinedmagnitudes. A detailed example of such an image power calculator 702will be described in greater detail below with reference to FIG. 8.

The image power calculator 702 can receive the uncorrected digitalsignal from various points in the low IF receiver front end. Forexample, the image power calculator 702 can be coupled to the input ofthe corrector. As will be described in greater detail below, theimplementation of the image power calculator 702 can vary depending uponwhere this uncorrected complex digital signal is received from.

The wideband power calculator 704 is configured to receive a complexdigital signal generate a second measure of power in the correctedcomplex digital signal at a wideband frequency range. As will bedescribed in greater detail below, the power such a wideband frequencyrange can be used as baseline of comparison to determine the presence orabsence of a blocking signal at the −IF location. As one example, thewideband power calculator 704 can be configured to determine a measureof power at a wideband frequency range approximately equal to +/−3 timesthe intermediate frequency (i.e., +/−3IF). In one such example the imagefrequency range has a value of less than 3 MHz wherein the widebandfrequency range has a value of at least six times the intermediatefrequency.

Again, a variety of different types of devices can be used to determinesuch a second measure of power. In one example, magnitude detector andan averaging filter are used to determine the second measure of power.In this example the magnitude detector serves to determine a magnitudeof the mixed corrected complex digital signal and the averaging filterserves to average the determined magnitudes. A detailed example of sucha wideband power calculator 704 will be described in greater detailbelow with reference to FIG. 9.

The wideband power calculator 704 can receive the complex digital signalfrom various points in the low IF receiver front end. For example, thewideband power calculator 704 can be coupled to the output of thecorrector. In that case the complex digital signal is a correctedcomplex digital signal. In other embodiments, the wideband powercalculator 704 can be coupled to the input of the corrector. In thatcase the complex digital signal is an uncorrected complex digitalsignal.

The comparator 706 is configured to compare the first measure of powerand the second measure of power. As such, the comparator 706 can beimplemented with any suitable digital comparator, including digitallogic comparators. In one embodiment the output of the comparator 706 isa digital value in dB representing the difference between the firstmeasure of power and the second measure of power. This digital value indB can then be passed to the control signal generator 708 and useddetermine when the corrector is enabled and disabled.

The control signal generator 708 is configured to receive the output ofcomparator 706 and determine when the corrector is enabled and disabledfrom that output. In one embodiment, the control signal generator 708can be configured to selectively enable the corrector by and comparingthe power difference to a first threshold value and enabling thecorrector when the power difference is beyond that threshold value. Sucha condition can indicate that significant power is in the imagefrequency range and/or is being removed by the operation of thecorrector. And this condition is highly indicative of a blocking signalbeing present at the −IF location.

As one example embodiment the control signal generator 708 can beconfigured to enable the corrector when the first power measure is atleast 3 dB greater than the second power measure.

In another embodiment, the control signal generator 708 can beconfigured to selectively enable the corrector by comparing the powerdifference to a first threshold value and enabling the corrector whenthe power difference moves above the first threshold value and disablingthe corrector when the power difference is moves below the secondthreshold value. In such an embodiment the use of two differentthreshold values can provide hysteresis in the enabling and disabling ofthe corrector. This hysteresis can prevent undesirable chatter in theenabling and disabling of the corrector.

As one example, the control signal generator 708 can be configured toenable the corrector when the power differences moves beyond at least 3dB and then disable the corrector only when the power difference dropsbelow 1 dB. Again, the use of such two different threshold values canprovide hysteresis in the enabling and disabling of the corrector andthus prevent undesirable chatter.

Turning now to FIG. 8, an exemplary image power calculator 800 isillustrated schematically. In general, the image power calculator 800 isconfigured to generate a first measure of power in the image frequencyrange of the complex digital signal (I+jQ). To facilitate this the imagepower calculator 800 includes a mixer 802, a low pass filter 804, amagnitude detector 806 and an averaging filter 808.

The mixer 802 is configured to receive the uncorrected complex digitalsignal and down mix signals from the image frequency range that cancontain −IF blocking signals to the baseband frequency. As such, themixer 802 can be implemented with any suitable digital mixer, such as aquadrature digital mixer. It should be noted that the mixer 802 can beconfigured to receive the uncorrected complex digital signals from avariety of different locations on the low IF receiver, and that theamount of frequency shift needed to shift the −IF blocking signals tothe baseband would depend upon the location the signals are obtainedfrom. For example, in the corrector 200 of FIG. 2 the uncorrectedcomplex digital signal can be obtained from before or after the complexconjugate operator 202. And because the complex conjugate operator 202changes the frequency distribution of the signal, the amount offrequency shift needed to shift the −IF blocking signals to basebandwould be different for signals received from those two different places.Thus, by implementing the appropriate mixing parameters the mixer 802can be configured to shift signals originating in the image frequencyrange that can contain −IF blocking signals to the baseband frequency.

The output of the mixer 802 is coupled to the low pass filter 804. Ingeneral, the low pass filter 804 is configured to reduce the bandwidthof the down mixed complex digital signal to facilitate sub-rate samplingof the complex digital signal. As such, the low pass filter 804 can beimplemented with any suitable digital low pass filter that facilitatessampling. In one embodiment, the low pass filter 804 can be implementedwith a digital “integrate and dump” method that integrates the incomingdigital signal and outputs the integrating result after a time intervalwhile resetting the integrator. This process is repeated to generate astream of averaged complex digital signal values that provide adecimated rate of information.

The output of the low pass filter 804 is coupled to the magnitudedetector 806. The magnitude detector 806 determines the magnitude of thefiltered complex digital signal values. It should be noted that thepower measurement does not generally require a precise determination ofsignal magnitude. As such, it can be desirable to use a moderatelyaccurate but relatively fast digital magnitude detector. Thus, themagnitude detector 806 can be implemented with any suitable digitalmagnitude approximation or other determination technique. For example,the digital magnitude detector can be implemented to use a techniquewhere the magnitude |I+jQ| is approximated as:

${{I + {j\; Q}}} \cong {{\max\left\{ {I,Q} \right\}} + {\frac{3}{8}\min\left\{ {I,Q} \right\}}}$where I and Q are the in-phase and quadrature components.

The output of the magnitude detector 806 is coupled to the averagingfilter 808. In general, the averaging filter 808 determines a runningaverage of the magnitude of the filtered complex digital signal values.This running average of the magnitudes can then be used as a firstmeasure of power in the image frequency range of the complex digitalsignal (I+jQ). A variety of different types of digital filters can beused to implement the averaging filter 808. In one example a single poledigital filter can be used. More specifically, an infinite impulseresponse (IIR) digital filter can be used. And again, the output of thesuch an IIR filter is the running average of the magnitude of thefiltered complex digital signal values that can be passed to acomparator (e.g., comparator 706 in FIG. 7) and used as a first measureof power in the image frequency range.

Turning now to FIG. 9, an exemplary wideband power calculator 900 isillustrated schematically. In general, the wideband power calculator 900is configured to generate a second measure of power in the widebandfrequency range of the complex digital signal (I+jQ). To facilitate thisthe wideband power calculator 900 includes a mixer 902, a magnitudedetector 904 and an averaging filter 906.

The wideband power calculator 900 is configured to receive the complexdigital signal. As noted above, this complex digital signal can be acorrected or uncorrected signal. Thus, in one embodiment, the mixer 902uses corrected signals, and thus it receives signals from the output ofthe corrector (e.g., at the output of corrector 108, 200, 400). Thus inthis embodiment the mixer 902 would receive signals that have alreadybeen mixed with the generated cancellation signal (e.g., after theoutput image canceller 208 in FIG. 8). In other embodiments, the mixer902 uses uncorrected signals, and thus can receive signals from theinput to the corrector (e.g., at the input of corrector 108, 200, 400)or from other locations.

In either case the mixer 902 is configured to down mix the receivedcomplex digital signal to the baseband frequency. As such, the mixer 902can be implemented with any suitable quadrature digital mixer. Forexample, the quadrature digital mixer can multiply the incoming signals(t) by:exp(j2πft)to achieve mixing to the baseband frequency, where f is the mixingfrequency and t is time.

The output of the mixer 902 is coupled to the magnitude detector 904.The magnitude detector 904 again determines the magnitude of thefiltered complex digital signal values. It should again be noted thatthe power measurement does not generally require a precise determinationof signal magnitude. As such, it can be desirable to use a moderatelyaccurate but relatively fast digital magnitude detector. Thus, themagnitude detector 904 can again be implemented with any suitabledigital magnitude technique.

The output of the magnitude detector 806 is coupled to the averagingfilter 906. In general, the averaging filter 906 again determines arunning average of the magnitude of the filtered complex digital signalvalues. This running average of the magnitudes can then be used as asecond measure of power in a wideband frequency range of the correctedcomplex digital signal (I+jQ) at the wideband frequency range. Again, avariety of different types of digital filters can be used to implementthe averaging filter 906. In one example a single pole digital filtercan be used. More specifically, an infinite impulse response (IIR)digital filter can be used. And again, the output of the such an IIRfilter is running average of the magnitude of the corrected complexdigital signal values that can be passed to a comparator (e.g.,comparator 706 in FIG. 7) and used as a second measure of power in thewideband frequency range.

The embodiments described herein thus provide systems and methods fordigital correction in low intermediate frequency (IF) receivers that canprovide improved performance. Specifically, the embodiments describedherein use digital correction techniques that can correct for signaldistortions in low IF receivers caused by I-Q imbalance, including bothI-Q magnitude imbalance and I-Q phase imbalance. In general, theembodiments described herein are implemented to at least partiallycancel an image of a blocking signal in the complex digital signal. Sucha cancellation can be implemented to at least partially cancel an imageof blocking signal where that image occurs at or near the intermediatefrequency (i.e., an image of an −IF blocking signal at occurs at the IFfrequency). As such, the embodiments described herein can be applied toa wide variety of radio frequency (RF) applications and protocols aspreviously listed.

In one embodiment, a low intermediate frequency (IF) receiver isprovided, the low IF receiver comprising: a mixer configured to becoupled to an antenna for receiving an RF signal at a fundamentalfrequency (f0), the mixer configured to generate I and Q signals at alow intermediate frequency from the received RF signal; an analog todigital converter configured to receive the I and Q signals and generatea complex digital signal (I+jQ), where the complex digital signal (I+jQ)includes an image of a blocking signal created by IQ imbalance in themixer, the image of the blocking signal having a frequency at or nearthe low intermediate frequency; a corrector configured to receive thecomplex digital signal (I+jQ) and at least partially cancel the image ofa blocking signal using a cancellation signal to generate a correctedcomplex digital signal (I+jQ); and a corrector controller configured toselectively enable the corrector based at least in part on a firstmeasure of power in the complex digital signal (I+jQ) at an imagefrequency range and a second measure of power in the complex digitalsignal (I+jQ) at a wideband frequency range.

In another embodiment, a method of processing a received radio frequency(RF) signal is provided, comprising: mixing the received RF signal togenerate I and Q signals at a low intermediate frequency; converting theand Q signals to a complex digital signal (I+jQ), where the complexdigital signal (I+jQ) includes an image of a blocking signal created byIQ imbalance during the mixing, the image of the blocking signal havinga frequency at or near the low intermediate frequency; training acorrector to generate a cancellation signal; and selectively enablingthe corrector based at least in part on a first measure of power in thecomplex digital signal (I+jQ) at an image frequency range and a secondmeasure of power in the complex digital signal (I+jQ) at a widebandfrequency range, where the corrector partially cancels an image of theblocking signal using the cancellation signal when enabled.

In another embodiment, a low intermediate frequency (IF) receiver isprovided, the low IF receiver comprising: a mixer configured to becoupled to an antenna for receiving an RF signal at a fundamentalfrequency (f0), the mixer configured to generate I and Q signals at alow intermediate frequency from the received RF signal; an analog todigital converter configured to receive the I and Q signals and generatea complex digital signal (I+jQ), where the complex digital signal (I+jQ)includes an image of a blocking signal created by IQ imbalance in themixer, the image of the blocking signal having a frequency at or nearthe low intermediate frequency; a corrector configured to receive thecomplex digital signal (I+jQ) and at least partially cancel the image ofa blocking signal using a cancellation signal to generate a correctedcomplex digital signal (I+jQ); a corrector controller configuredselectively enable and disable the corrector, the corrector controllerincluding: an image power calculator configured to generate a firstmeasure of power in the complex digital signal (I+jQ) at an imagefrequency range; a wideband power calculator configured to generate asecond measure of power in the complex digital signal (I+jQ) at awideband frequency range; a comparator configured to compare the firstmeasure of power and the second measure of power; and wherein thecorrector controller is configured to selectively enable and disable bycomparing a power difference between the first measure of power and thesecond measure of power to a first threshold and a second threshold, andenabling the corrector when the power difference is above the firstthreshold and disabling the corrector when the power difference is belowthe second threshold.

For the sake of brevity, conventional techniques related to signalprocessing, sampling, analog-to-digital conversion, digital-to-analogconversion, analog circuit design, differential circuit design, andother functional aspects of the systems (and the individual operatingcomponents of the systems) may not be described in detail herein.Furthermore, the connecting lines shown in the various figures containedherein are intended to represent exemplary functional relationshipsand/or physical couplings between the various elements. It should benoted that many alternative or additional functional relationships orphysical connections may be present in an embodiment of the subjectmatter. It should be understood that circuitry described herein may beimplemented either in silicon or another semiconductor material oralternatively by software code representation thereof.

As used herein, a “node” means any internal or external reference point,connection point, junction, signal line, conductive element, or thelike, at which a given signal, logic level, voltage, data pattern,current, or quantity is present. Furthermore, two or more nodes may berealized by one physical element (and two or more signals can bemultiplexed, modulated, or otherwise distinguished even though receivedor output at a common mode). The foregoing description refers toelements or nodes or features being “connected” or “coupled” together.As used herein, unless expressly stated otherwise, “connected” meansthat one element/node/feature is directly joined to (or directlycommunicates with) another element/node/feature, and not necessarilymechanically. Unless expressly stated otherwise, “coupled” means thatone element is directly or indirectly joined to (or directly orindirectly communicates with) another element, and not necessarilymechanically. Thus, although the schematics shown in the figures depictexemplary arrangements of elements, additional intervening elements,devices, features, or components may be present in an embodiment of thedepicted subject matter. In addition, certain terminology may also beused in the foregoing description for the purpose of reference only, andthus are not intended to be limiting.

The terms “first,” “second,” “third,” “fourth” and the like in thedescription and the claims are used for distinguishing between elementsand not necessarily for describing a particular structural, sequentialor chronological order. It is to be understood that the terms so usedare interchangeable under appropriate circumstances. Furthermore, theterms “comprise,” “include,” “have” and any variations thereof, areintended to cover non-exclusive inclusions, such that a circuit,process, method, article, or apparatus that comprises a list of elementsis not necessarily limited to those elements, but may include otherelements not expressly listed or inherent to such circuit, process,method, article, or apparatus.

The foregoing description of specific embodiments reveals the generalnature of the inventive subject matter sufficiently that others can, byapplying current knowledge, readily modify and/or adapt it for variousapplications without departing from the general concept. Therefore, suchadaptations and modifications are within the meaning and range ofequivalents of the disclosed embodiments. The inventive subject matterembraces all such alternatives, modifications, equivalents, andvariations as fall within the spirit and broad scope of the appendedclaims.

What is claimed is:
 1. A low intermediate frequency (IF) receiver, thelow IF receiver comprising: a mixer configured to be coupled to anantenna for receiving an RF signal at a fundamental frequency (f₀), themixer configured to generate I and Q signals at a low intermediatefrequency from the received RF signal; an analog to digital converterconfigured to receive the I and Q signals and generate a complex digitalsignal (I+jQ), where the complex digital signal (I+jQ) includes an imageof a blocking signal created by IQ imbalance in the mixer, the image ofthe blocking signal having a frequency at or near the low intermediatefrequency; a corrector configured to receive the complex digital signal(I+jQ) and at least partially cancel the image of the blocking signalusing a cancellation signal to generate a corrected complex digitalsignal (I+jQ); and a corrector controller configured to selectivelyenable the corrector based at least in part on a first measure of powerin the complex digital signal (I+jQ) at an image frequency range and asecond measure of power in the complex digital signal (I+jQ) at awideband frequency range.
 2. The low IF receiver of claim 1, wherein thecorrector controller further comprises a comparator configured tocompare the first measure of power and the second measure of power. 3.The low IF receiver of claim 1, wherein the corrector controller isconfigured to selectively enable the corrector based at least in part onthe first measure of power in the complex digital signal (I+jQ) at theimage frequency range and the second measure of power in the complexdigital signal (I+jQ) at the wideband frequency range by determining apower difference and comparing the power difference to a firstthreshold.
 4. The low IF receiver of claim 1, wherein the correctorcontroller is configured to selectively enable the corrector based atleast in part on the first measure of power in the complex digitalsignal (I+jQ) at the image frequency range and the second measure ofpower in the complex digital signal (I+jQ) at the wideband frequencyrange by determining a power difference and comparing the powerdifference to a first threshold and a second threshold, and wherein thecorrector controller is configured to enable the corrector when thepower difference is above the first threshold and to disable thecorrector when the power difference is below the second threshold. 5.The low IF receiver of claim 4, wherein the first threshold is greaterthan the second threshold by at least one dB.
 6. The low IF receiver ofclaim 1, wherein the image frequency range is a range of less than 3 MHzand wherein the wideband frequency range has is a range of at least sixtimes the intermediate frequency.
 7. The low IF receiver of claim 1,wherein the corrector controller further comprises: an image powercalculator configured to generate the first measure of power in thecomplex digital signal (I+jQ) at the image frequency range; and awideband power calculator configured to generate the second measure ofpower in the complex digital signal (I+jQ) at the wideband frequencyrange.
 8. The low IF receiver of claim 7, wherein the image powercalculator comprises: a mixer configured to mix the complex digitalsignal (I+jQ) at the image frequency range to a baseband frequency; alow pass filter configured to filter the mixed complex digital signal(I+jQ); a magnitude detector configured to determine a magnitude of themixed complex digital signal (I+jQ); and a first averaging filterconfigured to average the determined magnitude of the mixed complexdigital signal (I+jQ).
 9. The low IF receiver of claim 8, wherein thelow pass filter comprises an integrate and dump function and wherein thefirst averaging filter comprises an infinite impulse response (IIR)filter.
 10. The low IF receiver of claim 8, wherein the mixer is coupledto an input of a complex conjugate operator to receive the complexdigital signal (I+jQ).
 11. The low IF receiver of claim 8, wherein themixer is coupled to an output of a complex conjugate operator to receivethe complex digital signal (I+jQ).
 12. The low IF receiver of claim 7,wherein the wideband power calculator comprises: a mixer configured tomix the complex digital signal (I+jQ) at the wideband frequency range toa baseband frequency; a magnitude detector configured to determine amagnitude of the mixed complex digital signal (I+jQ); and a secondaveraging filter configured to average the determined magnitude of themixed complex digital signal (I+jQ).
 13. The low IF receiver of claim12, wherein the second averaging filter comprises an infinite impulseresponse (IIR) filter.
 14. The low IF receiver of claim 1, wherein thecomplex digital signal (I+jQ) at the wideband frequency range is thecorrected complex digital signal (I+jQ).
 15. The low IF receiver ofclaim 1, wherein the complex digital signal (I+jQ) at the widebandfrequency range is the complex digital signal (I+jQ) before thecancelling of the image of the blocking signal.
 16. The low IF receiverof claim 1, further comprising a trainer configured to train thecorrector to generate the cancellation signal.
 17. A method ofprocessing a received radio frequency (RF) signal, comprising: mixingthe received RF signal to generate I and Q signals at a low intermediatefrequency; converting the and Q signals to a complex digital signal(I+jQ), where the complex digital signal (I+jQ) includes an image of ablocking signal created by IQ imbalance during the mixing, the image ofthe blocking signal having a frequency at or near the low intermediatefrequency; training a corrector to generate a cancellation signal; andselectively enabling the corrector based at least in part on a firstmeasure of power in the complex digital signal (I+jQ) at an imagefrequency range and a second measure of power in the complex digitalsignal (I+jQ) at a wideband frequency range, where the correctorpartially cancels the image of the blocking signal using thecancellation signal when enabled.
 18. The method of claim 17, where theselectively enabling of the corrector based at least in part on thefirst measure of power in the complex digital signal (I+jQ) at the imagefrequency range and the second measure of power in the complex digitalsignal (I+jQ) at the wideband frequency range comprises determining apower difference and comparing the power difference to a firstthreshold.
 19. The method of claim 17, where the selectively enabling ofthe corrector based at least in part on the first measure of power inthe complex digital signal (I+jQ) at the image frequency range and thesecond measure of power in the complex digital signal (I+jQ) at thewideband frequency range comprises determining a power difference andcomparing the power difference to a first threshold and a secondthreshold, and enabling the corrector when the power difference is abovethe first threshold and disabling the corrector when the powerdifference is below the second threshold.
 20. A low intermediatefrequency (IF) receiver, the low IF receiver comprising: a mixerconfigured to be coupled to an antenna for receiving an RF signal at afundamental frequency (f₀), the mixer configured to generate I and Qsignals at a low intermediate frequency from the received RF signal; ananalog to digital converter configured to receive the I and Q signalsand generate a complex digital signal (I+jQ), where the complex digitalsignal (I+jQ) includes an image of a blocking signal created by IQimbalance in the mixer, the image of the blocking signal having afrequency at or near the low intermediate frequency; a correctorconfigured to receive the complex digital signal (I+jQ) and at leastpartially cancel the image of the blocking signal using a cancellationsignal to generate a corrected complex digital signal (I+jQ); acorrector controller configured selectively enable and disable thecorrector, the corrector controller including: an image power calculatorconfigured to generate a first measure of power in the complex digitalsignal (I+jQ) at an image frequency range; a wideband power calculatorconfigured to generate a second measure of power in the complex digitalsignal (I+jQ) at a wideband frequency range; a comparator configured tocompare the first measure of power and the second measure of power; andwherein the corrector controller is configured to selectively enable anddisable by comparing a power difference between the first measure ofpower and the second measure of power to a first threshold and a secondthreshold, and enabling the corrector when the power difference is abovethe first threshold and disabling the corrector when the powerdifference is below the second threshold.